The invention relates generally to electrical networks, and more specifically, to methods of synchronizing input signals into the electrical networks.
Electric devices may be connected or organized in a network to enable the transmission of power to the devices, or communication between the devices. Such a network of interconnected devices may be described as a grid. For example, an electric grid may be an interconnected network for delivering electricity from one or more power generators to the connected devices (e.g., customers of the utility company). A power grid may transmit AC power at a synchronized frequency, amplitude, and/or phase angle to efficiently connect a large number of power generators and devices. Synchronized operation of a grid, or portions of a grid, may enable a pooling of power generation, as well as a pooling of loads to result in lower operating costs.
The synchronized transmission of AC power may be beneficial for efficiently transmitting and/or distributing of power. However, many factors may disturb the synchronization of a grid. For example, voltage imbalances, angular frequency variations, and voltage harmonic distortions may significantly disturb grid synchronization. In particular, voltage imbalances may be common in a power grid, as single phase loads of a grid may not be evenly distributed between the phases of the supplied power and may be continuously connected and disconnected. Such discrepancies in the amplitudes, frequencies, and/or phase angles between two parallel voltages may cause abnormal current circulation within the grid which may result in a large current imbalance. Imbalanced currents may stress grid devices, such as AC-DC converters, cycloconverters, active filters, induction motors, and other energy storage systems which function to convert and/or transfer power through the grid to the connected electric devices. Imbalanced current may also stress grid link inductors and capacitors, and imbalanced current in one end device of a grid may introduce a torque ripple through the grid.
Power converters used in single-phase applications such as fundamental front end (FFE) regenerative braking may be even more susceptible to damages or inefficiencies resulting from imbalanced current, as the DC bus voltage ripple may become higher in single-phase applications than in three-phase applications due to the relatively higher input current and high DC bus capacitor ripple in single-phase applications. Furthermore, the drive may not be able to operate properly if the current circulating through the single-phase converter is not controlled.
Conventional methods of synchronizing a grid include using a phase lock loop (PLL) having a standard synchronous reference frame. However, such conventional methods may not be sufficient for alleviating the effects associated with an unbalanced grid, particularly the effects associated with single-phase converter applications. Methods of decreasing the effects of voltage and/or current imbalance in a grid that employs single-phase applications may improve the performance and synchronous operation of the grid.